2008-09-25_REPORT - C1981010

• Permit No: C1981010
• IBM Index Class Name: REPORT
• Doc Date: 9/25/2008
• Doc Name: Response/Review of July 2008 Stability Analysis Report Memo
• From: Marcia L Talvitie
• To: Daniel Hernandez, Jason D Musick
• Permit Index Doc Type: Stability Report
• Email Name: DIH
• Media Type: D
• Archive: No

STATE OF COLORADO
DIVISION OF RECLAMATION, MINING ANp SAFETY
Department of Natural Resources
1313 Sherman St., Room 215
Denver, Colorado 80203 COLORADO

Phone: [phone] D IV IS I ON OF

FAX: [phone] RECLAMATION
MINING

INTEROFFICE MEMORANDUM SAFETY
Bill Ritter, Jr.
Date: 25 September 2008 Governor
Harris D. Sherman
To: Daniel I. Hernandez
Executive Director
Jason D. Musick Ronald W. Cattany
Division Director

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Subject: Permit No. C-1981-010 - Trapper Mine
Review of July 2008 Stability Analysis Report
For Simplified K-Pit with Toe Buttress
In response to your request of 24-Aug-2008, I have conducted a review of the Stability Analysis
Report prepared by Agapito Associates, Inc. (AAI) in July 2008 for Trapper Mining, Inc. The
report presents determinations of Factor of Safety (F.S.) for the toe buttress and various
configurations of phreatic surface and spoil backfill in the reclaimed K-Pit at the Trapper Mine.
My comments on the report are listed below, for your consideration.
1) Two-dimensional numerical modeling was used (FLAC/Slope), incorporating rock and
spoil attributes derived from a 3D stability analysis of the G-Pit landslide. Given the
reported lack of variability (Item 2.1) in the geometry of the K-Pit, using a 2D analysis is
probably reasonable, as is generation of lithologic data from borehole 05-Gl-CCR.
2) Two groundwater conditions were analyzed: 1) with the phreatic surface along the pit
floor; and 2) with the phreatic surface at the top of the in-pit spoil. The last paragraph of
Item 2.5 includes the following: "...the phreatic surface was made to drop to follow the
L-seam floor to the base of the model. This assumption was based on field observations
made by TMI staff." A porosity of 5% was assumed for each bedrock layer, which
would include the L-Seam and the L-Seam Floor.
Please provide additional information to support the assumption that the L-Seam floor
(rather than the H, I, J, K, M, or Q Seam floor, etc.) will control the phreatic surface in
the K-Pit backfill, particularly in the case of the phreatic surface being located at the
top of the spoil backfill. If the potential does exist for the phreatic surface to develop at
an elevation higher than the L-Seam floor, please provide additional simulations
evaluating the F.S. values to be expected for the varying backfill depths.
This concludes my review of the report. Please let me know if you have any questions or
concerns.
cc: Sandy Brown
Office of Office of
Mined Land Reclamation Denver • Grand Junction • Durango Active and Inactive Mines